1. Field of the Invention
The invention relates to input displays and more particularly to touch sensitive displays.
2. Description of the Related Art
With an input display, a user can select from displayed options by directly touching the desired option display. Common techniques employed in an input display for detecting the location of a touch include infrared light, surface acoustic waves, optical sensing, electromagnetic sensing, capacitance sensing and resistance sensing.
Input displays comprise a touch panel supported above a glass screen of the displays for detecting touch position, thus, cost is increased, and display transmittance is reduced by about 20%. Embedded input displays use the optical sensitivity of amorphous silicon to integrate a photo sensor into array processes of thin-film transistor liquid crystal displays (TFT-LCD); thus, cost is lower and optical performance is better.
The photo sensor receives ambient light and the backlight reflected by the internal cell elements of the input display. Ambient light is desirable, but reflected backlight is considered to be noise by the photo sensor. The signal-to-noise ratio becomes smaller as ambient light becomes weaker or the reflected backlight becomes stronger.
In FIG. 1, a conventional input display is shown. The input display comprises a liquid crystal layer 7 sandwiched between glass substrates 4 and 6. A photosensitive thin film transistor 5 is disposed on a surface of the glass substrate 4 opposite to glass substrate 6, and a black matrix 10 and a color filter 20 are disposed on a surface of the glass substrate 6 opposite the glass substrate 4. The opening 11 of the black matrix and the opening 21 of the color filter 20 expose the channel region of the photosensitive thin film transistor 5, thus, the photosensitive thin film transistor 5 can detect ambient light entering through openings 11 and 21.
Representative light beams 1 through 3 from the backlight (not shown) of the input display are shown in FIG. 1. Light beam 1 is reflected by the interface of the liquid crystal layer 7 and the color filter 20, Light beam 2 is reflected by the interface of the color filter 20 and the black matrix 10, and Light beam 3 is reflected by the interface of the color filter 20 and the glass substrate 6. Reflected light beams 1 through 3 are transmitted into the channel region of the photosensitive thin film transistor 5, and can also be detected by the photosensitive thin film transistor 5. When an operator touches the input display and blocks ambient light from entering openings 11 and 21, the photosensitive thin film transistor 5 detects “dark information” and transmits an input signal. At the same time, however, the light beams 1 through 3, are continuously reflected into the photosensitive thin film transistor 5. The photosensitive thin film transistor 5 may detect the reflected light beams; thus, the input signal is hidden by the reflected light beams, causing input failure. This error occurs more frequently when ambient light is weak.